1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method that reduce jaggies in edge portions of image data performed digital halftoning.
2. Description of the Related Art
Conventionally, some techniques have been proposed for reducing notches called jaggies that occur in edge portions of characters or the like in an image processing apparatus. There are various reasons why jaggies can occur, based on the type of jaggies. Jaggies can be roughly divided into two kinds, stairs of pixels caused by a low-resolution printer and notches caused by digital halftoning such as screen processing.
For reducing the former stairs of pixels, there is, for example, a technique to match patterns on binary images so as to detect edges and to thereby add a pixel that corresponds to a pattern to a matched location or remove a pixel (see Japanese Patent Laid-Open No. 10-42141, for example). With this technique, the locations of jaggies are detected by pattern matching and then smoothing processing is implemented on edge portions by adding data obtained as a result of dividing a single pixel into multiple pixels in the case of a binary printer, or by adding an intermediate level of dots in the case of a multi-value printer.
For reducing the latter notches, there is, for example, a technique for generating correction data from image data performed pre-digital halftoning and then adding the correction data to edge portions of image data performed digital halftoning so as to fringe the edge portions (see Japanese Patent Laid-Open No. 2006-295877, for example). With this technique, jaggies caused by screen processing are reduced by determining whether or not to smoothing processing is to be performed on an edge portion and then, if the edge portion necessitates smoothing processing, comparing the correction data and the image data performed digital halftoning and outputting the data with the higher value. This technique further concurrently reduces the former stairs of pixels caused by a low-resolution printer, despite its simple architecture.
The above-described technique, however, poses the problem of adverse effects caused by the use of attribute data at the time of determining whether or not to perform smoothing processing on an edge portion, the attribute data representing the attributes of objects included in image data, such as characters, lines, figures, and images. In other words, the determination of an edge portion according to the attribute data means determining an edge portion where objects (attributes) are switched, so that in some cases smoothing processing may be performed on a boundary between objects of the same color and different attributes that is not regarded as an edge portion from the pixel values of the objects. For example, FIG. 17 illustrates the result of smoothing processing performed on contone (an abbreviation for “continuous tone”) image data illustrated in FIG. 12A, attribute data illustrated in FIG. 10B, and halftone image data illustrated in FIG. 12B, using the technique of Japanese Patent Laid-Open No. 2006-295877. Referring to the attribute data in FIG. 10B, “I” represents the attribute of an image and “G” represents the attribute of a graphic, and it can be seen from FIG. 17 that there is a problem in that smoothing processing adds correction data to the boundary between the image and the graphic.